Moisture sensors are increasingly found in regulating devices, in particular in air moisteners. They have also been used in soil moisture regulation, more specifically in greenhouses where the air and the soil must be simultaneously maintained at fixed levels of moisture. Generally, the moisture sensors found in industrial greenhouses are sophisticated and expensive devices monitored by computers or full time operators in connection with watering devices or humidifiers. These devices are activated when the moisture level falls below a predetermined level by means of appropriate alarms and/or software. Other types of industrial monitoring unrelated to plant watering needs are also sophisticated devices, such as U.S. Pat. No. 4,350,040.
Some prior art also exists for monitoring the watering needs of house plants. These devices measure soil impedance changes across probes placed proximate to the plant. As soil moisture changes, the impedance changes until a set point is reached and an indicator signals the need for watering, as shown in U.S. Pat. No. 4,020,417. Some of these monitors are related to or have been developed from the sophisticated devices used in industrial applications. These sophisticated devices may provide a means to precisely and linearly calibrate the device (such as taught by U.S. Pat. No. 4,621,229) and/or include precise temperature compensators (such as disclosed in U.S. Pat. No. 3,961,247). Also as a carry-over from the industrial grade equipment, an audible alarm is also used an an indicator of watering needs.
In contrast to the precision, availability of power, operators and automatic devices in an industrial application, house plant monitoring devices are typically battery powered, precision and temperature compensation is unnecessary and full time operators are not present. Battery life is a key consideration in the in-home application. Although, the majority of the time, the current drain on the battery is expected to be caused by the monitoring of the soil probes, the current draw of the watering need indicating means must also be limited to prolong battery life since the house may be empty for extended periods.
The need for extended battery life in these in-home applications has lead to sophisticated sampling and monitoring circuits to minimize the monitoring drain on the battery, such as taught in U.S. Pat. No. 4,514,722. LED indicators, piezoelectric ceramic beepers, and DC to DC converters are also employed to minimize current draw (U.S. Pat. No. 4,514,722). These approaches have increased battery life, but have resulted in a sophisticated device.
What is needed is a small, low cost, in-home monitor of soil moisture, not an adaptation of industrial grade equipment. The power requirements and irritating audible alarms of industrial grade equipment also may not be appropriate for the home environment.
None of the prior art the applicant is aware of incorporates a monitoring circuit that draws less than 5 microamps or provides a pleasant indicator of watering needs. The prior art current drain results in the need for sophisticated sampling circuits or space consuming larger batteries in order to obtain a battery life in excess of one year.